The present invention relates to structures, by which part of the incoming high-frequency energy can be separated to its own path or energies coming from different paths can be combined to a common path. Means like this are needed in units connected to the base station antennas of mobile networks, for example.
High-frequency dividing means include power dividers and directional couplers. In a power divider, the incoming energy is divided to two or more output paths so that the powers of the branches are usually equally high. A common divider type is the Wilkinson divider, by which the energy can be divided to several output paths as matched and with relatively small losses. The directional coupler has four ports: The energy coming to the input port is mostly directed to a second port, a relatively small part of the incoming energy is directed to the third port, and hardly any energy goes to the fourth port.
In practice, the dividing means are mostly realized by using microstrips. FIG. 1 shows an example of such a prior art structure. This is a four-branch Wilkinson divider, which is manufactured in an ordinary circuit board. The circuit board includes a dielectric board 101, on the lower surface thereof a conductor plane 102 connected to the signal ground, and on the upper surface a microstrip 103. The characteristic impedance of the transmission line formed by these parts is Zo, which is the same as the impedance of the feed line of the structure. The strip 103 is branched into four microstrips 111, 112, 113 and 114. Their length is xcex/4 at the operating frequency, and each of them forms an impedance Zo/4=Zo/2 with the board 101 and the ground plane 102. A discrete resistor 121, the resistance of which is Zo, is connected to the second end of the microstrip 111. Correspondingly, similar resistors 122, 123 and 124 are connected to the second ends of the strips 112, 113 and 114, respectively. The second ends of the resistors are connected together with a conductor 105, which consists of three jumper wires. If a multilayer board were used, a strip inside the board 101 would correspond to the conductor 105. The microstrip 111 continues from the connecting point of the resistor 121 onward as a narrower microstrip 131, which forms an impedance Zo with the board 101 and the ground plane 102. The microstrip 131 leads to the first output out1. The strips 112, 113 and 114 continue in the same way. They lead to the outputs out2, out3 and out4. The structure has the drawback that the connecting of the discrete components requires joints on the board, which means reduced reliability.
A structure corresponding to that shown in FIG. 1 can also be implemented by thin-film technology, whereby the resistive components are formed by sputtering, for example. A structure like this has the drawback that its costs, including encapsulation, are relatively high.
A simple directional coupler can be made by arranging another conductor in parallel with the signal strip conductor on the surface of a dielectric board, the other side of which acts as the ground plane. This structure has the drawback that its directional properties are relatively poor. A structure with better directional properties is obtained when both strips are arranged inside a dielectric board, both sides of which are ground planes. A tighter electromagnetic coupling compared to both structures is obtained e.g. by the so-called Lange coupler. FIG. 2 shows the Lange coupler in the prior art form. It has three conductor areas on the surface of a dielectric board. The first conductor area comprises a quarter-wave long, strip-like center conductor 201, a first strip extension 202 and a second strip extension 203. The extensions 202 and 203 reach from the opposite ends of the structure to the middle of the center conductor 201. The ends of the extensions are connected with conductor wires 221 and 222 to the midpoint of the center conductor. The second conductor area comprises a quarter-wave long strip conductor 211, which runs beside the center conductor, between it and the first extension 202. The third conductor area comprises a quarter-wave long strip conductor 212, which runs beside the center conductor, between it and the second extension 203. The center conductor 201 remains between the conductor strips 211 and 212. The conductor strips 211 and 212 are connected to each other with conductor wires 223 and 224 at the opposite ends of the structure. The structure is a four-port. Port 1 is linked with the end of the conductor 211, which is not between the extension 202 and the center conductor. Port 2 is linked with the end of the conductor 212, which is not between the extension 203 and the center conductor. Port 3 is linked with the branching point of the center conductor and the extension 203. Port 4 is linked with the branching point of the center conductor and the extension 202. Each port also includes the ground plane, which is not drawn in FIG. 2. The signal is fed to port 1, for example. Then most of the energy fed in comes out from port 2. Part of the incoming energy is transferred to port 3. This part is relatively small. Instead, hardly any energy is transferred to port 4. The drawback of the Lange coupler is the joints required by the jumper wires, which mean reduced reliability and an increase in manufacturing costs. In addition, the surface area required is relatively large, because the conductor strips are placed on the same level.
The purpose of the invention is to reduce the above mentioned drawbacks of the prior art. The means according to the invention is characterized in what is set forth in the independent claim. Some preferred embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is the following: All parts of the dividing means are integrated into a monolithic structure in an insulating material, preferably multilayer ceramics. The transmission line strips and other conductors are formed by printing conductive material on the outer surface of the ceramic piece and in its interlayers, when required. The conductors between the surfaces are formed by filling the hole made through the layer or layers with conducting material. The resistive components parallel with and between the surfaces are formed in a similar manner.
The invention has the advantage that the dividing means becomes reliable. Another advantage of the invention is the fact that the manufacturing costs of the dividing means are relatively low. Both of these advantages are due to the monolithic structure, in which no wire joints are needed. Yet another advantage of the invention is the fact that the structure according to it can be fitted in a relatively small space, because structural parts can be placed on top of each other in the insulating material, and also vertically inside the board. Furthermore, the invention has the advantage that the transmission lines, in which the TEM (transversal electromagnetic) wave, which is advantageous for the coupling, propagates, can be manufactured in a relatively simple manner.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.